1. Field of the Invention
The present invention relates to a method for temperature compensation of a digital pressure meter, more particularly one, according to which a low cost temperature compensation circuit is fitted to a low cost digital pressure meter such that the digital pressure meter can perform accurate pressure measurement under different surrounding temperatures.
2. Brief Description of the Prior Art
Referring to FIG. 1, a conventional digital pressure meter includes a power circuit 10, a pressure sensor 20, an operational amplifier (OP Amp) 30, an analog-to-digital converter 40, a microprocessor 50, a liquid crystal display (LCD) driver circuit 60, a LCD 601, and a buzzer 501.
The power circuit 10 supplies fixed voltage source or electric current source to the pressure sensor 20, which is used for measuring pressure, and which will produce analogue signals according to the measurement. The operational amplifier 30 is used for amplifying the analogue signals of the pressure sensor 20. The analog-to-digital converter 40 is used for converting the amplified analogue signals to digital ones, which will be processed by means of the microprocessor 50. Then, the measured value is displayed on the LCD 601. And, the buzzer 501 is used for alarming and reminding the user.
Referring to FIG. 2, a circuit diagram of the pressure sensor 20, the pressure sensor 20 includes four piezoresistors 202 to 205 connected together to form a Whetstone bridge 201. The resistance of the piezoresistors 202 and 204 will decrease as the measured value of pressure increases. And, the resistance of the piezoresistors 203 and 205 will increase as the measured value of pressure increases. When the power circuit gives voltage to the pressure sensor 20 through junctions 206 and 207 of Whetstone bridge 201, analogue signals will be produced according to voltage difference between junctions 208 and 209 as well as variation of the measured value of pressure.
Although electronic pressure meters are more accurate and stable than mechanical ones, piezoresistor type pressure sensors can be affected more easily to have large errors by temperature than mechanical ones. Therefore, many manufacturers of piezoresistor type pressure sensors, e.g. NovaSensor, and Sensortechnics, would often equip pressure sensors with temperature compensation circuits such that the values of pressure measured by the pressure sensors will be maintained correct even if temperature of the pressure sensors changes.
There are two major types of temperature compensation circuits. Referring to FIG. 3, a thermistor circuit 70 is arranged between the power circuit 10 and the pressure sensor 20. Because the resistance of the piezoresistors of the pressure sensor 20 will increase as temperature increases, the temperature coefficient of output span will decrease as temperature increases, and a thermistor of negative temperature coefficient (NTC) has to be used; thus, the resistance of the thermistor will decrease as temperature increases, allowing electric current passing through the pressure sensor 20 to increase to compensate for the temperature coefficient of output span. Consequently, the pressure sensor 20 will always provide correct measured values of pressure under different temperatures.
Referring to FIG. 4, a diode circuit 80 is arranged between the power circuit 10 and the pressure sensor 20. Because the resistance of the piezoresistors of the pressure sensor 20 will increase as temperature increases, the temperature coefficient of output span will decrease as temperature increases, and a diode of negative temperature coefficient (NTC) has to be used; thus, the forward voltage of the diode will reduce as temperature increases, allowing electric current passing through the pressure sensor 20 to increase to compensate for the temperature coefficient of output span. Consequently, the pressure sensor 20 will always provide correct measured values of pressure under different temperatures.
From the above description, it can be understood that conventionally, the voltage of power of a pressure sensor is applied to temperature compensation for the pressure sensor, by means of which compensation the temperature effect on span is reduced to the smallest extent possible. However, the above temperature compensation circuits will cause reduction to the voltage of power of pressure sensors, and in turns, voltage output and sensitivity of the pressure sensors will reduce. Consequently, amplifiers have to be used for effecting large amplification ratio of the voltage output, which will also amplify another parameter, Zero Pressure Offset, causing reduction to the correctness of the values of pressure measured by the pressure sensors. Therefore, in order to obtain more correct measured values of pressure, another compensation circuit has to be used for compensation for Temperature Effect On Offset once the above temperature compensations are used.